In the case of observing an subject or processing a workpiece, light output from a light source such as a laser light source is focused and irradiated onto the subject or workpiece through an irradiating optical system including lenses etc. It is known that the beam waist diameter being a measure of the size of the focusing diameter when thus focused light can be reduced only to about a half of the wavelength of light. This is referred to as a diffraction limit. However, the diffraction limit is a matter in terms of Gaussian mode (or fundamental mode) light. On the other hand, the existence of high-order mode lights having more minute spatial structures than the diffraction limit is known.
Known as light beams having such properties are Bessel beams, Laguerre-Gaussian beams (hereinafter, referred to as “LG beams”), and polarization mode beams. Using such light beams allows effectively focusing the energy of light into a minute area smaller than the diffraction limit. In the past, there have been proposed, on the basis of this principle, inventions such as pickup devices, micro-processing technologies, and microscopes having resolutions smaller than the diffraction limit using Bessel beams.
Moreover, light source devices that output LG beams have been described in, for example, Non-Patent Documents 1 to 6. The light source devices described in these documents generate LG beams whose phases change along the circumferential direction in light beam cross sections. Such LG beams are expected to be applied to optical tweezers, quantum computations and quantum communications, etc., and have currently been attracting attention in the optical and physical fields.    Non-Patent Document 1: J. Arlt, et al., Journal of Modern Optics, Vol. 45, No. 6, pp. 1231-1237 (1998).    Non-Patent Document 2: D. G Grier, Nature, Vol. 424, pp. 810-816 (2003).    Non-Patent Document 3: M. W. Beijersbergen, et al., Optics communications, Vol. 112, pp. 321-327 (1994).    Non-Patent Document 4: K. Sueda, et al., Optics Express, Vol. 12, No. 15, pp. 3548-3553 (2004).    Non-Patent Document 5: N. R Heckenberg, et al., Optics Letters, Vol. 17, No. 3, pp. 221-223 (1992).    Non-Patent Document 6: N. R. Heckenberg, et al., Optical and Quantum Electronics, Vol. 24, No. 24, pp. 155-166 (1992).    Non-Patent Document 7: K. S. Youngworth and T. G Brown, Optics Express, Vol. 7, No. 2, pp. 77-87 (2000).    Non-Patent Document 8: R. Oron, et al., Applied Physics Letters, Vol. 77, No. 21, pp. 3322-3324 (2000).